1. Field of the Invention
This invention relates to a process for the ambient temperature grinding of soft polymers in the presence of a grinding aid.
2. State of the Art
It is often desired to produce polymer powders or small granules for use in polymer compounding, especially for blending with other polymers to modify or improve their properties. A typical example of such an application is the use of an ethylene/vinyl acetate/carbon monoxide terpolymer as a modifier of polyvinyl chloride, as described in U.S. Pat. No. 3,780,140 to Hammer.
Grinding of soft and sticky polymers at ambient temperature often is difficult or even impossible because of the reagglomeration of the small particles produced into larger aggregates and because of sticking of the material to the grinding equipment. One way to solve this problem is to grind soft polymers using liquid nitrogen as the cooling medium. This process, known as cryogenic grinding, is quite expensive. In addition, it is practical only in equipment of a rather limited size, which can process only a relatively small amount of polymer per unit of time.
Another prior art method calls for the addition to the polymer of a grinding aid, which adheres to the surfaces of the polymer particles and thus reduces their tackiness and prevents or reduces their reagglomeration into larger aggregates. See, for example, U.S. Pat. Nos. 3,190,565 to Jayne and 3,648,937 to Ehrreich; Japanese patent application No. 79/52,154 to Nitto Electric Industries; and Japanese patent application No. 80/109,614 to Matsushita Electric Works. Grinding in the presence of a powdered organic polymeric additive or of an inorganic powder is carried out at room temperature. This process is of interest when the presence of the grinding aid is not detrimental to the final product.
Grinding at ambient temperature in the presence of a grinding aid has some drawbacks, which make this technique impractical for commercial production. First, the grinding rate is quite slow, and thus the throughput of the material often is unsatisfactory. Second, the process requires elaborate dust collection and recycling equipment because a large proportion of the grinding aid is not effectively attached to the polymer particles and must be recovered. There is a need, therefore, for an ambient temperature grinding process, where those prior art drawbacks would be eliminated.